Image processing apparatus, image processing method, and storage medium

ABSTRACT

An object is to enable highly accurate density unevenness correction while suppressing a reduction in productivity of printing accompanying correction value calculation for density unevenness correction. In the image processing apparatus, density correction information that specifies an output tone value for implementing a target density for an input tone value for each nozzle and which does not include the influence by a non-ejectable nozzle that cannot eject ink normally is acquired. In a case where a non-ejectable nozzle is detected during printing processing, output tone values corresponding to the detected non-ejectable nozzle and peripheral nozzles thereof among output tone values specified in the density correction information are changed.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an image processing technique forreducing density unevenness and streaks that occur at the time offorming an image by ejecting ink.

Description of the Related Art

Conventionally, an ink jet printing apparatus is used that forms adesired image on a printing medium by ejecting ink droplets from eachnozzle while relatively moving a print head having a nozzle row in whicha plurality of ink ejection ports (nozzles) is arrayed and the printingmedium. There is a case where the ink head that is used in the ink jetprinting apparatus has a variation in the ejection amount among aplurality of nozzles due to manufacturing errors and the like. In a casewhere the variation in the ejection amount such as this exists, it mayhappen sometimes that density unevenness occurs in an image that isformed.

Conventionally, as processing to reduce the density unevenness such asthis, the HS (Head Shading) technique is known. In the HS technique, thedensity unevenness that occurs in a formed image is reduced byincreasing or reducing the number or the size of ink dots that arefinally formed in accordance with information (nozzle characteristic)relating to the ejection amount of each nozzle. At the time of acquiringthe above-described nozzle characteristic, for example, a method is usedgenerally in which a chart including patches of uniform density for eachtone is printed on a paper surface and the printing results are read bya scanner and the read image is analyzed.

On the other hand, there is a case where a non-ejectable nozzle thatcannot eject ink droplets occurs among a plurality of nozzles within theprint head. As a technique to suppress white streaks on an image, whichresult from the non-ejectable nozzle such as this, non-ejectioncomplementation processing is known. In the non-ejection complementationprocessing, white streaks are suppressed by forming ink droplets thatshould be formed by the non-ejectable nozzle by another nozzle in acomplementary manner. Japanese Patent Laid-Open 2012-71474 has describeda technique to complement ink droplets in the charge of thenon-ejectable nozzle by peripheral nozzles thereof based on the acquirednozzle characteristic.

The density unevenness correction processing and the non-ejectioncomplementation processing, both described above, are processingindependent of each other, but it is known that as a result of bothpieces of processing being performed in the area corresponding to thenon-ejectable nozzle and the peripheral nozzles thereof in anoverlapping manner, the correction in the area becomes excessive and ablack streak and density unevenness occur. In this regard, JapanesePatent Laid-Open No. 2012-147126 has described a technique to suppressblack streaks and density unevenness by modifying the read data of thedensity distribution measurement chart based on the information on thenon-ejectable nozzle and calculating the correction value for thedensity unevenness correction based on the modified read data.

There is a case where a non-ejectable nozzle recovers and becomes anormal nozzle by performing cleaning processing of the print head in themaintenance mode or the like. On the other hand, there is a case where anon-ejectable nozzle occurs suddenly during execution of printingprocessing. Because of this, in a case where an attempt is made tomaintain favorable printing results by the method of Japanese PatentLaid-Open No. 2012-147126 described above, it is necessary to frequentlyperform correction value calculation for the density unevennesscorrection that takes the non-ejectable nozzle into consideration.However, the above-described correction value calculation requires muchtime and effort, and therefore, in a case where this is performedfrequently, productivity of printing is reduced.

Consequently, an object of the technique of the present disclosure is toenable highly accurate density unevenness correction while suppressing areduction in productivity of printing accompanying correction valuecalculation for density unevenness correction.

SUMMARY OF THE INVENTION

The image processing apparatus according to the present disclosure is animage processing apparatus for performing printing on a printing mediumby using a print head including a plurality of nozzles ejecting ink, andincludes: a detection unit configured to detect a non-ejectable nozzlethat cannot eject ink normally among the plurality of nozzles; anacquisition unit configured to acquire density correction informationthat specifies an output tone value for implementing a target densityfor an input tone value for each nozzle and which does not includeinfluence by the non-ejectable nozzle; and a changing unit configured tochange output tone values corresponding to the detected non-ejectablenozzle and peripheral nozzles thereof among output tone values specifiedin the density correction information based on results of detection bythe detection unit.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a hardware configuration of an image formingsystem:

FIG. 2 is a diagram showing a configuration example of a print head:

FIG. 3 is a diagram showing a function configuration of an imageprocessing module;

FIG. 4 is a flowchart showing a flow of non-ejectable nozzle detectionprocessing;

FIG. 5A and FIG. 5B are each a diagram showing an example of anon-ejectable nozzle detection chart;

FIG. 6 is a flowchart showing a flow of density correction informationgeneration processing according to a first embodiment;

FIG. 7 is a diagram showing an example of a density characteristicacquisition chart;

FIG. 8 is a flowchart showing a flow of correction table creationprocessing;

FIG. 9A and FIG. 9B are each a diagram explaining a derivation method ofa correction value for an input tone value;

FIG. 10 is a flowchart showing a flow of printing processing in theimage forming system;

FIG. 11A and FIG. 11B are diagrams explaining a change of a correctiontable;

FIG. 12 is a flowchart showing a flow of correction processing:

FIG. 13 is a flowchart showing a flow of correction value distributionratio determination processing;

FIG. 14 is a diagram showing an example of a distribution ratioacquisition chart;

FIG. 15 is a diagram showing an example of a distribution ratio inaccordance with a tone value;

FIG. 16 is a diagram showing a way of thinking in a third embodiment;

FIG. 17A is a diagram showing an example of a non-ejection influenceacquisition chart and FIG. 17B is a diagram showing an example of acharacteristic of a non-ejection influence template:

FIG. 18 is a diagram showing a relationship between FIGS. 18A and 18B,and FIGS. 18A and 18B are flowcharts showing a flow of generalprocessing in an image forming system according to the third embodiment;

FIG. 19 is a diagram showing an example of a non-ejection influencetemplate:

FIG. 20A is a diagram showing a nozzle row characteristic beforeapplying a non-ejection influence template and FIG. 20B is a diagramshowing an example of the nozzle row characteristic after applying thenon-ejection influence template:

FIG. 21 is a diagram explaining a fourth embodiment;

FIG. 22 is a flowchart showing a flow of printing processing accordingto a fifth embodiment; and

FIG. 23 is a flowchart showing details of pixel value exchangeprocessing.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, with reference to the attached drawings, the presentinvention is explained in detail in accordance with preferredembodiments. Configurations shown in the following embodiments aremerely exemplary and the present invention is not limited to theconfigurations shown schematically.

First Embodiment

In the present embodiment, a correction table not including theinfluence of a non-ejectable nozzle is obtained by repeating cleaningprocessing of a print head and outputting of a predetermined chart towhich a pattern capable of detecting a non-ejectable nozzle is attacheduntil a non-ejectable nozzle is no longer detected by using thepredetermined chart. Then, the occurrence of white streak due to anon-ejectable nozzle is suppressed by performing non-ejectable nozzledetection processing at predetermined timing in a case of performingprinting processing and appropriately modifying the correction table ina case where a non-ejectable nozzle is detected.

<Hardware Configuration of Image Forming System>

FIG. 1 is a diagram showing the hardware configuration of an imageforming system 10 according to the present embodiment. The image formingsystem 10 in the present embodiment has an image processing apparatus11, an image forming apparatus 12, an input device 13, a display device14, and an external storage device 15. In the following, components ofthe image forming system 10 are explained.

The image processing apparatus 11 comprises a CPU 100, a RAM 101, a ROM102, an image processing module 106, an external I/F (interface) 110,and a bus 111 and functions as a so-called image processing controller.Further, the image processing apparatus 11 is connected to the inputdevice 13, the display device 14, and the external storage device 15 viathe external I/F 110.

The CPU (Central Processing Unit) 100 controls the operation of theentire image forming system 10 by using input data and computer programsstored in the RAM 101 and the ROM 102. Here a case is explained as anexample where the CPU 100 controls the entire image forming system 10,but it may also be possible to control the entire image forming system10 by a plurality of pieces of hardware sharing the processing.

The RAM (Random Access Memory) 101 temporarily stores computer programsand data read from the external storage device 15 and data received formthe outside via the external I/F 110. Further, the RAM 101 is used as astorage area that is used in a case where the CPU 100 performs variouskinds of processing and a storage area that is used in a case where theimage processing module 106 performs image processing. The ROM (ReadOnly Memory) 102 stores setting parameters, boot programs and the likeof each unit within the image processing apparatus 11.

The image processing module 106 is implemented by a processor capable ofexecuting computer programs and a dedicated image processing circuit andperforms various kinds of image processing for converting image datathat is input as a printing target into image data that can be output bythe image forming apparatus 12. The configuration may be one in whichthe CPU 100 performs various kinds of image processing as the imageprocessing module 106 in place of preparing a dedicated processor as theimage processing module 106.

The external I/F 110 is an interface for connecting the image processingapparatus 11 and the image forming apparatus 12, the input device 13,the display device 14, and the external storage device 15. Further, theexternal I/F 110 also functions as a communication interface forperforming transmission and reception of data with an external device,not shown schematically, by using infrared communication, a wirelessLAN, the internet or the like.

The input device 13 is, for example, a keyboard, a mouse and the likeand receives an operation (instruction) by an operator. It is possiblefor an operator to input various instructions to the CPU 100 via theinput device 13. The display device 14 is, for example, a CRT, a liquidcrystal display and the like, and displays processing results by the CPU100 by images, characters and the like. In a case where the displaydevice 14 is a touch panel capable of detecting a touch operation, itmay also be possible for the display device 14 to function as part ofthe input device 13.

The external storage device 15 is, for example, a large-capacityinformation storage device, such as an HDD and an SSD. In the externalstorage device 15, the OS, computer programs for causing the CPU 100 toperform various kinds of processing, data and the like are saved.Further, the external storage device 15 also stores various tables andthe like, in addition to storing image data that is input and output andtemporary data that is generated by the processing of each unit. Forexample, a color conversion table used in the image processing module106, a threshold value matrix, information relating to the ink injectionof each nozzle, image data of each chart for density characteristicacquisition and non-ejectable nozzle detection, and the like are stored.Computer programs and various kinds of data stored in the externalstorage device 15 are read appropriately in accordance with the controlby the CPU 100 and stored in the RAM 101 and become a target of theprocessing by the CPU 100.

The image forming apparatus 12 comprises a printing module 107, an imagesensor 108, a maintenance module 109, an external I/F 112, a bus 113,and a RAM 114.

The external I/F 112 is an interface for connecting the image formingapparatus 12 to the image processing apparatus 11. The RAM 114 is usedfor storage of data and the like being processed and temporarily stores,for example, image data (halftone image data) for print output, which isacquired from the image processing apparatus 11. The printing module 107forms an image on a printing medium by the ink jet method based on thehalftone image data stored in the RAM 114. The halftone image data isacquired directly from the image processing module 106 of the imageprocessing apparatus 11 or by reading it from the external storagedevice 15. The print head comprised by the printing module 107 hasnozzle rows (printing element columns) corresponding to the number ofink colors, in which a plurality of ink ejectable nozzles (printingelements) is arrayed in one direction. FIG. 2 is a diagram showing aconfiguration example of the print head. In a case of an image formingsystem compatible with color printing, typically, the print head mountsfour nozzle rows corresponding to each ink of cyan (C), magenta (M),yellow (Y), and black (K). In FIG. 2 , for simplification ofexplanation, only the nozzle row of black (K) is shown schematically.The print head shown in FIG. 2 is a long line head that covers theentire width of a drawing area in the direction (x-direction) parallelto the nozzle row. The print head forms an image on a printing medium bygenerating dots by ejecting ink droplets while relatively moving theprinting medium in the direction (y-direction) perpendicular to thenozzle row, which is perpendicular to the direction parallel to thenozzle row based on a drive signal. In FIG. 2 , that the nozzle whosenozzle position number is 7 has become a non-ejectable nozzle isindicated by a x mark. In the present specification, it is assumed thatthe term “non-ejectable nozzle” generally means a nozzle in the state ofnot capable of ejecting ink normally and includes a nozzle not capableof ejecting an appropriate amount of ink to an appropriate position. Inaddition to a nozzle in the state of not capable of ejecting ink at allbecause of clogging or the like.

The image sensor 108 is a sensor for capturing an image formed on aprinting medium by the printing module 107 and the image sensor 108 is,for example, a line sensor and an area sensor. The image sensor 108functions as a unit configured to detect a non-ejectable nozzle from acaptured image and a unit configured to acquire the ink ejectioncharacteristic of each nozzle. It is not necessary for the image sensor108 to be provided within the image forming apparatus 12 and forexample, the image sensor 108 may be an in-line scanner or an offlinescanner, not shown schematically, which is connected via the externalI/F 110 of the image processing apparatus 11.

The maintenance module 109 performs cleaning processing for recoveringthe print head comprised by the printing module 107 by removing nozzleclogging. At the method of cleaning processing, for example, there is amethod of moving the print head up to a position at which an absorbentmaterial (sponge or the like) of wasted ink is located and forcing theink head to eject a predetermined amount of ink from each nozzle withinthe nozzle row. Further, there is a method of pushing out ink forciblyby applying a pressure from the side of an ink tank. Alternatively,there is a method of removing clogging by forcibly sucking ink byapplying a negative pressure from the outside of the nozzle. It isassumed that the image forming apparatus 12 of the present embodimentcomprises an automatic cleaning mechanism by one of the above-describedmethods.

<Function Configuration of Image Processing Module 106>

Next, by using FIG. 3 , the function configuration of the imageprocessing module 106 is explained. The image processing module 106 hasa color conversion processing unit 301, a correction processing unit302, an HT processing unit 303, a non-ejectable nozzle detection unit304, and a density correction information generation unit 305. Theresolution of the image data that is handled in the image processingmodule 106 is the same as the resolution of the nozzle arrangement ofthe print head and for example, 1,200 dpi. In the following, each unitis explained.

The color conversion processing unit 301 converts the input image datafrom the external storage device 15 into image data corresponding to thecolor reproduction area of the printing module 107. In the presentembodiment, the input image data is 8-bit image data indicating colorcoordinates (R, G, B) in the color space coordinates, such as sRGB thatare the representation colors of a monitor. The color conversionprocessing unit 301 converts the 8-bit input image data of each of RGBinto 8-bit image data of each of R′ G′ B′ corresponding to the colorreproduction area of a printer. For the conversion, it is possible touse a publicly known method, such as matrix calculation processing andprocessing using a three-dimensional lookup table. Further, the colorconversion processing unit 301 performs conversion processing to convertthe 8-bit image data of each of R′ G′ B′ after the conversion into colorsignals corresponding to a plurality of inks used in the printer. In acase where the printing module 107 uses, for example, the inks of black(K), cyan (C), magenta (M), and yellow (Y), conversion is performed into8-bit image data of each of CMYK. This color conversion is alsoperformed by using a three-dimensional lookup table together withinterpolation calculation as in the above-described conversion from RGBinto R′G′B′. As another conversion method, it is also possible to use amethod, such as matrix calculation processing, as in the above.

The correction processing unit 302 performs correction processing forreducing both the density unevenness due to the difference in the nozzlecharacteristic and the white streak due to a non-ejectable nozzle forthe image data of each color plane of CMYK after the color conversionprocessing based on the density correction information on each nozzle.Details of the correction processing will be described later.

The HT processing unit 303 performs conversion into the number tonesthat the printing module 107 can represent and halftone processing fordetermining the dot arrangement for the image data after the correctionprocessing or multi-tone image data stored in the external storagedevice 15. The HT processing unit 303 of the present embodiment convertsimage data in which one pixel is represented by eight bits into 1-bitbinary halftone image data (output image data) in which each pixel has avalue of “0” or “1”. In the halftone image data, the pixel whose pixelvalue (output value) is “0” indicates off of a dot and the pixel whosepixel value (output value) is “1” indicates on of a dot. To the halftoneprocessing, it is possible to apply a publicly known method, such as theerror diffusion method and the dither method. The halftone image datagenerated by the halftone processing is sequentially delivered directlyto the printing module 107 within the image forming apparatus 12 or viathe RAM 101 or the external storage device 15.

The non-ejectable nozzle detection unit 304 specifies a non-ejectablenozzle (its nozzle position number) in which an ink ejection failure hasoccurred in each nozzle row based on printing results of a non-ejectablenozzle detection chart that is output from the printing module 107. Thenon-ejectable nozzle detection chart is printed and output for each inkcolor (that is, for each nozzle row). For example, in a case where theprinting module 107 uses four kinds of ink of CMYK, the non-ejectablenozzle detection chart is output for each ink and a non-ejectable nozzleis specified for each color of CMYK (for each nozzle row). Details ofthe processing to detect a non-ejectable nozzle will be described later.The processing contents are common to each ink color, and therefore, inthe following, explanation is given by taking the nozzle row of the Kink as an example.

The density correction information generation unit 305 generates densitycorrection information that specifies, for each nozzle configuring thenozzle row, an output tone value (density correction value) for an inputtone value, with which the density unevenness is reduced in the printingresults based on the scanned image obtained by reading the densitycharacteristic acquisition chart. Here, in the density characteristicacquisition chart, at least a patch of uniform density that is foracquiring the characteristic of each nozzle and in which the density isvaried stepwise is included. This density characteristic acquisitionchart is also printed and output for each ink color. For example, in acase where the printing module 107 uses the four kinds of ink of CMYK,the dedicated chart is output for each ink color and the correctionvalue is derived for each nozzle in the nozzle row of each of CMYK andthe density correction information is generated. Details of the densitycorrection information generation processing will be described later.The processing contents of the density correction information generationprocessing are common to each ink color (each nozzle row) like thenon-ejectable nozzle detection processing, and therefore, in thefollowing, explanation is given by taking the nozzle row of the K ink asan example.

The components of the image processing module 106 are not limited tothose described above. For example, it may also be possible toadditionally comprise a dedicated function unit configured to performeach piece of processing, such as generation of distribution ratioinformation in the second embodiment, creation of a non-ejectioninfluence template in the third embodiment, and pixel value exchange inthe fifth embodiment.

<Non-Ejectable Nozzle Detection Processing>

Following the above, details of the processing to detect a non-ejectablenozzle in each nozzle row, which is performed by the non-ejectablenozzle detection unit 304, are explained with reference to the flowchartin FIG. 4 .

First, at S401, data of a non-ejectable nozzle detection chart image forwhich halftone processing has been performed is acquired from theexternal storage device 15 and transmitted to the printing module 107along with printing instructions thereof. The printing module 107 havingreceived the printing instructions forms the chart image on a sheet andoutputs the sheet. FIG. 5A shows an example of the non-ejectable nozzledetection chart image. The chart image in FIG. 5A has a configuration of16 vertical pixels×16 horizontal pixels and the numerical value (“0” or“255”) of each pixel indicates the tone value. Further, the figures from0 to 15 attached at the top of the chart image each indicate the nozzleposition number corresponding to each pixel column and it is assumedthat the correspondence between this nozzle position number and thenozzle position number in each nozzle row comprised by the print headshown in FIG. 2 is maintained. In the non-ejectable nozzle detectionchart, a rectangle (including four pixels in the example in FIG. 5A) inthe shape of a line is arranged so that it is possible to determinewhether or not ink is ejected for each nozzle.

Next, at S402, the non-ejectable nozzle detection chart that is outputfrom the printing module 107 is read by the image sensor 108.

Then, at S403, based on the read image (scanned image) of thenon-ejectable nozzle detection chart obtained at S402, the position of anon-ejectable nozzle is specified. FIG. 5B is a diagram schematicallyshowing a scanned image in a case where the nozzle whose nozzle positionnumber is 7 is detected as a non-ejectable nozzle. As shown in FIG. 5B,in a case w % here a non-ejectable nozzle exists, the line that shouldbe originally formed at the nozzle position is not formed. By causingthe line that is not formed to correspond to the nozzle position numberin this manner, it is possible to specify the position of thenon-ejectable nozzle in each nozzle row.

The above is the contents of the non-ejectable nozzle detectionprocessing.

<Density Correction Information Generation Processing>

Next, details of the processing to generate a correction table used inthe correction processing for suppressing density unevenness, which isperformed by the density correction information generation unit 305, areexplained with reference to the flowchart in FIG. 6 . Here, explanationis given by taking a case as an example where as density correctioninformation, a correction table in an LUT (lookup table) form isgenerated, in which a plurality of input tone values that vary stepwiseand correction values (output tone values) are associated with eachother. However, the LUT form is an example and this is not limited anddensity correction information may be accepted in which the correctionvalue for a certain input tone value is determined by using amathematical formula or a function.

First, at S601, data of a density characteristic acquisition chart imagefor which halftone processing has been performed is acquired from theexternal storage device 15 and transmitted to the printing module 107along with printing instructions thereof. The printing module 107 havingreceived the printing instructions forms the density characteristicacquisition chart image on a sheet and outputs the sheet. FIG. 7 showsan example of the density characteristic acquisition chart according tothe present embodiment. The density characteristic acquisition chartimage in the present embodiment includes two kinds of image area, thatis, a non-ejection detection area 701 and a density patch area 702. Thenon-ejection detection area 701 is an image area for detecting thenozzle position number in a case where there is a non-ejectable nozzleand for example, may be the same chart as the non-ejectable nozzledetection chart described previously. The density patch area 702 is animage area for acquiring the density characteristic of each nozzle ineach nozzle row configuring the print head. In the density patch area702 shown in FIG. 7 , nine kinds of rectangular patch of uniform densityin which the density is varied at nine levels are formed.

Next, at S602, the density characteristic acquisition chart that isoutput from the printing module 107 is read by the image sensor 108. Thecolor space of the read image (scanned image) of the densitycharacteristic acquisition chart is arbitrary, but here, it is assumedthat the read image is an image of three channels of RGB. Then, it isassumed that the scanned image of the three channels of RGB isconfigured into a scanned image of one channel by a color conversiontable prepared in advance in accordance with the reading characteristicof the image sensor 108. Here, the color conversion table is a tablethat converts the pixel value of the image into a value linear todensity, for example, such as the Y value in the CIEXYZ color space andthe L* value in the CIEL*a*b* color space. Further, in a case where eachpatch on the printed and output chart is formed by a color ink, such ascyan, magenta, and yellow, it may also be possible to use a valuecorresponding to saturation in place of a value corresponding tobrightness. For example, it may also be possible to use RGB values asvalues corresponding to the complementary colors of cyan, magenta, andyellow, respectively. In the present embodiment, it is assumed that thescan resolution is 1,200 dpi the same as the resolution of the nozzlearrangement of the print head.

Next, at S603, the non-ejection detection area 701 on the read image(scanned image) obtained at S602 is analyzed and in a case where anon-ejectable nozzle is detected, the nozzle position thereof isspecified. In a case where a non-ejectable nozzle is detected, thenozzle position number of the nozzle is stored in the external storagedevice 15 as non-ejectable nozzle information. In a case where thenon-ejection detection area 701 is detected from the scanned image, itmay be possible to apply a publicly known method, for example, such as apattern matching method and a method that uses a position marker (notshown schematically).

At S604, based on the analysis results of the non-ejection detectionarea at S603, the next processing contents are determined. In a casewhere a non-ejectable nozzle is detected as a result of the analysis,the processing advances to S605 and in a case where no non-ejectablenozzle is detected, the processing advances to S606.

At S605, the maintenance module 109 is instructed to perform cleaningprocessing for recovering the print head. Then, in the maintenancemodule 109 having received the instructions, the cleaning processing isperformed. At the same time as that, the scanned image data of thededicated chart that is read at S602 is discarded. In a case where thecleaning processing is completed, the processing returns to S601 and theprocessing at S601 to S604 is repeated again. That is, each time thedensity characteristic acquisition chart is output, an attempt is madeto recover the non-ejectable nozzle by performing the cleaningprocessing. It may also be possible to design a configuration in whichthe number of times the cleaning processing at 605 is performed iscounted and in a case where the count value exceeds a predeterminednumber of times, a user is notified of an error and the densitycorrection information generation processing is not performed until, forexample, the print head is exchanged with another.

At S606, processing to create a correction table excluding the influenceof a non-ejectable nozzle is performed by calculating the correctionvalue corresponding to the input tone value for each nozzle based on thescanned image acquired at S602. FIG. 8 is the flowchart showing detailsof correction table creation processing. In the following, explanationis given along the flow in FIG. 8 .

First, at S801, from the scanned image of the density characteristicacquisition chart acquired at S602, the density patch area 702 isdetected. At S802 that follows, from the detected density patch area702, a measured curve corresponding to each nozzle is acquired. Here,the measured curve is a curve indicating a correspondence relationshipbetween the tone value corresponding to each nozzle and the measuredsignal value on the scanned image. FIG. 9A shows an example of themeasured curve. In FIG. 9A, the horizontal axis represents the inputsignal value (input tone value) of the density patch area 702 and thevertical axis represents the signal value (to be strict, the value afterthree channels of RGB are converted into one channel indicating density.In the following, described as “measured value”) that is measured fromthe scanned image. Further, in FIG. 9A, a broken line 901 indicates theupper limit value of the horizontal axis and in a case of an 8-bit inputsignal value, the upper limit value is “255”. Then, a curve 902 in FIG.9A indicates a measured curve obtained by the measured valuecorresponding to each tone value of the density patch area 702 and theresults of interpolation calculation thereof. In the present embodiment,as the interpolation method, section linear interpolation is used.However, the interpolation method may be any one and it may also bepossible to use a publicly known spline curve. The measured curverepresents the density characteristic of the nozzle corresponding to apixel position x and is obtained corresponding to the number of nozzlesused at the time of forming the density characteristic acquisition chartimage on the sheet. A different measured curve is obtained for eachnozzle and for the nozzle whose ink ejection amount is small, themeasured curve shifts in the upward direction (in the direction in whichbrightness becomes higher) and for the nozzle whose ink ejection amountis large, the measured curve shifts in the downward direction (in thedirection in which brightness becomes lower). At next S803, a targetcharacteristic corresponding to each nozzle is acquired. Here, thetarget characteristic is the target density characteristic determined inadvance in accordance with the measured curve of each nozzle. A straightline 903 (a set of measured values that are linear to the tone) in FIG.9A indicates the target characteristic.

Then, at S804, based on the measured curve acquired at S802 and thetarget characteristic acquired at S803, the correction value inaccordance with each tone value is determined for each nozzle. FIG. 9Bis a diagram explaining how to find the correction value. First, thenozzle position number of the derivation-target nozzle of interest andthe input tone value for which it is desired to find the correctionvalue are acquired. In FIG. 9B, a point 904 on the horizontal axisindicates the input tone value. Next, the target density valuecorresponding to the input tone value 904 is found from the targetcharacteristic 903. In FIG. 9B, a point 905 on the vertical axisindicates the target density value that is found from the input tonevalue 904 and the target characteristic 903. Then, from the measuredcurve 902 of the nozzle of interest, the tone value corresponding to thetarget density value 905 is found and determined as the correction value(output tone value) corresponding to the input tone value 904. In FIG.9B, a point 906 on the horizontal axis indicates the correction value(output tone value) 906 corresponding to the input tone value 904. Byperforming the processing such as this for a plurality of tone valuesdetermined in advance, a correction table for the nozzle of interest isobtained in which the output tone value (correction value) is associatedwith a predetermined input tone value. In place of finding eachindividual correction value corresponding to all the input tone valuesfrom 0 to 255, it may also be possible to find only the correctionvalues corresponding to representative tone values (for example, ninetone values corresponding to the density patches). In that case, at thetime of performing the correction processing using the correction table,for the input tone value that is not specified within the table, it issufficient to find the corresponding correction value by interpolationprocessing.

In a case where the above-described processing is completed for all thenozzles of each nozzle row, the created correction table is stored inthe external storage device 15 and this processing is terminated.

The above is the contents of the density correction informationgeneration processing. It is necessary to complete the creation of thecorrection table before the start of execution of the printingprocessing based on user instructions and the creation of the correctiontable is performed at the time of shipment of the system or at the timeof attachment of the print head. Further, the creation or updating ofthe correction table is performed at predetermined timing designated bya user, such as the timing at which the print head is exchanged withanother. Alternatively, it may also be possible to evaluate thecorrection table at arbitrary timing and update the correction table ina case where the correction value deviates from the appropriatecorrection value due to a change in the nozzle characteristic or thelike.

<Flow of Printing Processing>

Following the above, the flow of the printing processing in the imageforming system 10 is explained along the flowchart shown in FIG. 10 . Inthe process of this printing processing, the correction processing usingthe correction table created by the above-described method is performed.Before giving a detailed explanation along the flow in FIG. 10 , thedesign concept of the printing processing according to the presentembodiment is explained.

The white spot resulting from a non-ejectable nozzle is likely to beconspicuous perceptually compared to the density unevenness due to adifference in the nozzle characteristic. Because of this, in a casewhere the white spot occurs, it is preferable to detect it quickly andperform complementation. Further, in order to detect a non-ejectablenozzle, it is only required to determine whether or not ink is ejected,and therefore, the number of dedicated charts to be output is small andthe processing time required for non-ejectable nozzle detection is shortcompared to those of the density correction information generationprocessing. On the other hand, the density unevenness results from thevariation in the ejection characteristic (ejection amount/ejectiondirection/ejection speed) at the time of manufacturing of the printhead, the inclination of the head at the time of attachment of the printhead, the crosstalk at the time of the drive of the print head, and thelike, and the density unevenness is unlikely to change over time.However, the characteristic of each nozzle is not linear to the inputlevel, and therefore, the number of dedicated charts to be outputincreases and the processing time required for derivation of thecorrection value is prolonged compared to those of the non-ejectablenozzle detection. Because of this, it is desirable to reduce theexecution frequency of the density correction information generationprocessing whose processing load is large as low as possible. On theother hand, as regards the non-ejectable nozzle detection processingwhose processing load is small, it is preferable to maintain the printedimage quality by increasing the execution frequency thereof. Theprinting processing designed based on the above-described basic conceptis performed based on user instructions. The series of processing shownin the flow in FIG. 10 is started by a user designating the file name ofprinting-target image data and the number of sheets to be printed, andgiving instructions to perform the processing via the input device 13.

First, at S1001, printing preparation processing is performed in theimage processing apparatus 11. Specifically, first, printing-targetimage data is read from the external storage device 15 based on the filename designated by a user and sent to the image processing module 106,and color conversion processing is performed in the color conversionprocessing unit 301. Further, a number of sheets to be output Ndesignated by a user is set to the RAM 101 or the like and further, acounter Cn_p that counts the number of printed and output sheets isinitialized (count value=0). In a case where the printing preparationprocessing is completed, the processing advances to S1002.

At S1002, the non-ejectable nozzle detection processing describedpreviously (see the flow in FIG. 4 ) is performed. The results of thenon-ejectable nozzle detection processing are saved in the externalstorage device 15 as non-ejectable nozzle information.

At S1003 that follows, based on the non-ejectable nozzle informationgenerated at S1002, the correction value corresponding to thenon-ejectable nozzle in the correction table is changed. Specifically,among the correction values of each tone value specified in associationwith the nozzle position number in the correction table, the correctionvalues corresponding to the found non-ejectable nozzle and theperipheral nozzles thereof are changed so that the white spot thatoccurs due to the non-ejectable nozzle becomes unlikely to be perceived.Here, a more detailed explanation is given by using a specific example.FIG. 11A shows the correction table before being changed, which has beenobtained by the density correction information generation processing. Inthe correction table in FIG. 11A, correction values corresponding tonine tones x number of nozzles are stored. For example, in a case wherethe input tone value of a printing-target image is “32”, the correctionvalue (output tone value) corresponding to the nozzle position number 1is “34”. Here, it is assumed that the nozzle whose nozzle positionnumber is n is detected as a non-ejectable nozzle in the non-ejectablenozzle detection processing at S1002 described above. FIG. 11B shows thecorrection table after being changed at this time. As shown in FIG. 11B,in the correction table after being changed, all the correction valuesof the nth nozzle, which is the non-ejectable nozzle, are changed to“0”. Changing the correction values of a certain nozzle to “0” meansthat control is performed so that the nozzle does not eject ink. Bychanging the correction values in this manner, it is possible tosuppress a black streak from occurring even in a case where thenon-ejectable nozzle recovers unexpectedly during the printingprocessing. Further, in the example shown in FIG. 11B, correction valuesI′ of the (n+1)th nozzle and the (n−1)th nozzle are changed to I+Ix/2.Here, I represents the correction value before being changed and Ixrepresents the correction value before being changed of the nth nozzlethat has become the non-ejectable nozzle. However, in a case where I′exceeds the maximum value (in the example in FIG. 11A and FIG. 11B, 255)of the tone value, I′ is clipped to the maximum value. The correctionvalues are changed so that the density that has originally been in thecharge of the non-ejectable nozzle is made up for by the peripheralnozzles adjacent to the non-ejectable nozzle. As a result of that, thenumber of dots or the dot size on the periphery of the non-ejectablenozzle increases, and thereby, it is possible to suppress a white spotcaused by the non-ejectable nozzle. In a case of clipping the correctionvalue I′ after being changed to the maximum value because the correctionvalue I′ exceeds the maximum value of the tone value, it may also bepossible to allocate the tone value corresponding to the clipped valueto the non-ejectable nozzle. In this case, even on the condition thatthe non-ejectable nozzle recovers naturally, dots are generated only inthe portion in which density is short, and therefore, a black streakdoes not occur. On the other hand, the non-ejectable nozzle is drivenwithout being masked, and therefore, the possibility of natural recoverybecomes strong.

Next, at S1004, for the printing-target image after the colorconversion, the correction processing using the changed correction tableis performed in the correction processing unit 302. Details of thecorrection processing will be described later. At S1005 that follows,for the corrected printing-target image, the halftone processing isperformed in the HT processing unit 303. Then, the generated halftoneimage data is sent to the image forming apparatus 12 via the externalI/Fs 110 and 112.

Next, at S1006, in the printing module 107 of the image formingapparatus 12, printing using the halftone image data received from theimage processing apparatus 11 is performed and the image designated by auser is formed on a sheet. At this time, the value of the counter Cn_pdescribed previously is incremented (+1). In a case where one sheet isprinted and output, the processing advances to S1007.

Then, at S1007, whether or not printing of the number of sheets to beoutput N, which is set at S1001, is completed is determined.Specifically, whether or not the value of the counter Cn_p is equal tothe value of the number of sheets to be output N is determined. In acase where the value of the counter Cn_p is equal to the value of thenumber of sheets to be output N, this printing processing is terminated.On the other hand, in a case where the value of the counter Cn_p is notequal to the value of the number of sheets to be output N, theprocessing advances to S1008.

At S1008, whether or not the value of the counter Cn_p has reached apredetermined number of sheets determined in advance is determined.Here, the predetermined number of sheets that is used as a thresholdvalue is, for example, a multiple of 200 or the like. In a case wherethe value of the counter Cn_p has reached the predetermined number ofsheets, the processing returns to S1002 and the detection of anon-ejectable nozzle, the change of the correction table, and thecorrection processing based on the changed correction table areperformed again. On the other hand, in a case where the value of thecounter Cn_p has not reached the predetermined number of sheets, theprocessing returns to S1006 and printing is continued.

The above is the contents of the printing processing according to thepresent embodiment. By performing the non-ejectable nozzle detectionprocessing each time a predetermined number of sheets is printed, it ispossible to deal with even a case without a break where a non-ejectablenozzle occurs during printing. Further, the correction table is createdin advance so that the influence of a non-ejectable nozzle is notincluded, and therefore, in a case where the non-ejectable nozzlerecovers, it is possible to correct the density that is in the charge ofthe nozzle and the peripheral nozzles without the need to create thecorrection table again.

It is premised that the density correction information generationprocessing to create the correction table is performed before the startof the printing processing, but it is not necessary to perform thedensity correction information generation processing each timer prior tothe input of printing instructions by a user and it is sufficient toperform the density correction information generation processing attiming at which a predetermined time elapses or a predetermined numberof processed sheets is reached. Alternatively, it may also be possibleto perform the density correction information generation processingbased on a predetermined event, such as exchange of the print head andturning on/off of the electric power source of the image forming system.Further, in a case where the number of sheets to be output that isdesignated is very large, it may also be possible to perform the densitycorrection information generation processing as interrupt processing atthe pint in time at which a predetermined time (for example, two hours)elapses or a predetermined number of processed sheets (for example,1,000 sheets) is reached.

<Density Correction Processing>

Next, details of the above-described correction processing (S1004) areexplained by taking a case as an example where the changed correctiontable shown in FIG. 11B is used. FIG. 12 is a flowchart showing a flowof processing in the correction processing unit 302. In the following, adetailed explanation is given along the flow in FIG. 12 .

First, at S1201, the position of the pixel of interest in the image dataafter color conversion, which is the processing target, is initialized.Due to this, for example, the pixel at coordinates (x, y)=(0, 0) in theimage after color conversion is determined as the first pixel ofinterest.

Next, at S1202, a nozzle position number x corresponding to the pixel ofinterest is acquired. For example, in a case where the dot at theposition of the pixel of interest (x, y)=(1, 1) is formed by the nozzlewhose nozzle position number is 1, the corresponding nozzle positionnumber x=0 is acquired.

Next, at S1203, from the image data after color conversion, a tone valuei of the pixel of interest is acquired. At S1204 that follows, acorrection value (output tone value) i′ of the pixel of interest isdetermined based on the changed correction table. Specifically, thecorrection value i′ corresponding to the nozzle position number xacquired at S1202 and the tone value i acquired at S1203 is determinedwith reference to the changed correction table. Here, it is assumed thatthe tone value i of the pixel of interest is 32. Here, the determinationis performed m accordance with the correction table in FIG. 11B, andtherefore, in a case where the nozzle position number x is 1, thecorrection value i′ is 34 and in a case where the nozzle position numberx is n, the correction value i′ is 0. In a case where the correspondingtone value does not exist within the correction table, for example, suchas a case where i=48, it is sufficient to determine i′ to be 52 byperforming linear interpolation processing.

Next, at S1205, whether or not the correction value is determined forall the pixels in the image data after color conversion, which is theprocessing target, is determined. In a case where the correction valueis already determined for all the pixels, this correction processing isterminated. On the other hand, in a case where a pixel for which thecorrection value is not determined yet exists, the processing advancesto S1206 and the position of the pixel of interest is updated. After theupdating, the processing returns to S1202 and the determination of thecorrection value for the new pixel of interest is continued.

The above is the contents of the correction processing based on thecorrection table.

As above, in the present embodiment, the output of the densitycharacteristic acquisition chart is performed a plurality of times untilthe condition that the non-ejectable nozzle is no longer detected issatisfied. By repeatedly performing the chart output until thenon-ejectable nozzle is no longer detected in this manner, theappropriate correction values for all the nozzles configuring the nozzlerow are obtained and the correction table not including the influence ofthe non-ejectable nozzle is created. The created correction table doesnot include the influence of the non-ejectable nozzle, and therefore,even in a case where the non-ejectable nozzle recovers, it is notnecessary to create the correction table again, leading to suppressionof the downtime, saving of the ink and the sheet required for output ofa dedicated chart.

Second Embodiment

In the first embodiment, by distributing the correction value of anon-ejectable nozzle to the correction values of the nozzles (in thefollowing, described as “adjacent nozzles”) adjacent to thenon-ejectable nozzle from left and right, it is made possible tosuppress both the density unevenness and the white spot due to anon-ejectable nozzle. In the second embodiment, more appropriate densitycorrection processing is implemented by determining in advance adistribution ratio of the correction value in accordance with the tonevalue of a non-ejectable nozzle and storing the distribution ration in aform of a table or the like. Explanation of the contents common to thoseof the first embodiment, such as the system basic configuration, isomitted or simplified and in the following, contents of distributionratio determination processing, which is the feature of the presentembodiment, are explained mainly.

<Distribution Ratio Determination Processing>

In the present embodiment, following the density correction informationgeneration processing described previously, processing to determine acorrection value distribution ratio is performed by a distribution ratioinformation generation unit (not shown schematically) within the imageprocessing module 106. In the following, a detailed explanation is givenalong the flowchart shown in FIG. 13 .

First, at S1301, data of a distribution ratio acquisition chart imagefor which halftone processing has been performed is acquired from theexternal storage device 15 and transmitted to the printing module 107along with printing instructions thereof. The printing module 107 havingreceived the printing instructions forms the distribution ratioacquisition chart image on a sheet and outputs the sheet. FIG. 14 showsan example of a distribution ratio acquisition chart according to thepresent embodiment. The distribution ratio acquisition chart image inthe present embodiment includes two kinds of image area, that is, anon-ejection detection area 1401 and a distribution ratio derivationarea 1402. The non-ejection detection area 1401 is an image area fordetecting the nozzle position number in a case where there is anon-ejectable nozzle and for example, may be the same chart as thenon-ejectable nozzle detection chart described previously. Thedistribution ratio derivation area 1402 is an image area for determiningan optimum distribution ratio for each tone. In the distribution ratioderivation area 1402 shown in FIG. 14 , for the uniform-density patchesof different tones, “0” is allocated to the tone value of the nozzle ofa predetermined nozzle position number intentionally at predeterminedintervals so that a dot is not formed in the nozzle whose tone value is“0”. In the following, the nozzle of the nozzle position number whosetone value is set to “0” intentionally is called “intentionalnon-ejectable nozzle”. Further, in the adjacent nozzles adjacent to theintentional non-ejectable nozzle from left and right, the tone valuesare changed in different distribution ratios (here, at four levels). Inthis manner, in the distribution ratio acquisition chart, for therectangular patch of uniform density, in which the density is madedifferent at a plurality of levels, non-ejectable nozzles are caused toappear intentionally at predetermined intervals.

Next, at S1302, a distribution ratio acquisition chart that is outputfrom the printing module 107 is read by the image sensor 108. The colorspace of the read image (scanned image) of the distribution ratioacquisition chart is, for example, RGB, that is, the image is an imageof three channels of RGB and it is assumed that the image is convertedinto a scanned image of one channel by a color conversion table preparedin advance.

Next, at S1303, the non-ejection detection area 1401 on the read image(scanned image) obtained at S1302 is analyzed and in a case where anon-ejectable nozzle is detected, the nozzle position thereof isspecified. In a case where a non-ejectable nozzle is detected, thenozzle position number of the nozzle is stored in the external storagedevice 15 as non-ejectable nozzle information.

At S1304, based on the analysis results of the non-ejection detectionarea at S1303, the next processing contents are determined. In a casewhere a non-ejectable nozzle is detected as a result of the analysis,the processing advances to S1305 and on the other hand, in a case whereno non-ejectable nozzle is detected, the processing advances to S1306.

At S1305, the maintenance module 109 is instructed to perform cleaningprocessing for recovering the print head. Then, in the maintenancemodule 109 having received the instructions, the cleaning processing isperformed. At the same time as that, the scanned image data of thededicated chart read at S1302 is discarded. In a case where the cleaningprocessing is completed, the processing returns to S1301 and theprocessing at S1301 to S1304 is repeated again. That is, each time thedistribution ratio acquisition chart is output, an attempt is made torecover the non-ejectable nozzle by performing the cleaning processing.It may also be possible to design a configuration in which the number oftimes the cleaning processing at S1305 is performed is counted and in acase where the count value exceeds a predetermined number of times, auser is notified of an error and the distribution ratio determinationprocessing is not performed until, for example, the print head isexchanged with another.

At S1306, based on the scanned image acquired at S1302, the distributionratio in accordance with the tone value is determined. In thedetermination of the distribution ratio, for example, a distributionratio is selected for each tone value, with which the density becomesthe most uniform in a case where a publicly known spatial filter isapplied to the scanned image. At this time, depending on the kind ofprinting medium to be used (plain paper, glossy paper, mat paper and thelike) or the characteristic of ink, an appropriate distribution ratio isdifferent. As a publicly known spatial filter, it may be possible to usea Gaussian filer, a VTF filter corresponding to the perceptionsensitivity of human eyes, and the like. A graph in FIG. 15 shows arelationship between the distribution ratio and the tone value. In thegraph in FIG. 15 , the horizontal axis represents the tone value, thevertical axis represents the distribution ratio, and a curve 1501represents how the distribution ratio changes in accordance with thetone value. In a case where the value of the distribution ratio is, forexample, “2.0”, the tone value corresponding to the density that is tobe in the charge of the non-ejectable nozzle is allocated to theadjacent nozzles adjacent to the non-ejectable nozzle from left andright, respectively. The distribution ratio thus determined is stored inthe external storage device 15 as the distribution ratio in the LUT formin which, for example, the input tone value and the distribution ratioare associated with each other. As in the case of the correction tablethat is used in the density correction processing, it may also bepossible to find only the distribution ratios corresponding torepresentative tone values in place of finding each individualdistribution ratio corresponding to all the input tone values between 0and 255. In that case, for the input tone value that is not specifiedwithin the table, it is sufficient to find the correspondingdistribution ratio by interpolation processing.

The above is the contents of the processing to determine thedistribution ratio of the correction value. In the example shown in FIG.15 , there is one curve representing a relationship between thedistribution ratio and the tone value. The reason is that the results ofaveraging the distribution ratios obtained from each of all thenon-ejectable nozzles provided intentionally are taken as thedistribution ratio common to all the nozzles. However, this is notlimited and for example, it may also be possible to find thedistribution ratio by taking all the nozzles as the non-ejectablenozzles intentionally and find the distribution ratio for each nozzle.Alternatively, it may also be possible to group the nozzles according totheir features and take the distribution ratio averaged for each groupas the distribution ratio information. For example, in a case where theprint head includes a combination of a plurality of nozzle rows, it mayalso be possible to perform grouping fore each nozzle row.Alternatively, it may also be possible to put the nozzles whose nozzleposition number is the same among a plurality of nozzle rows into thesame group.

Further, in the example described above, the distribution ratio isdetermined from the scanned image of the output dedicated chart, butthis is not limited. For example, a configuration may be accepted inwhich an operator visually checks the output distribution ratioacquisition chart and selects a distribution ratio with which theinfluence of a non-ejectable nozzle is the most unlikely to be perceivedfrom among a plurality of alternatives, or an operator directly inputsor sets a distribution ratio expected to be unlikely to be perceived.

<Change of Correction Table>

In a case of the present embodiment, in the change (S1003) of thecorrection table of the printing processing shown in FIG. 10 describedpreviously, the correction values of the non-ejectable nozzle and thenozzles adjacent thereto are changed as follows. Here, it is assumedthat the nozzle whose nozzle position number is n is detected as anon-ejectable nozzle in the non-ejectable nozzle detection processing(S1002). At this time, as the correction values corresponding to thenozzle whose nozzle position number is n, “0” is stored newly for allthe input tone values. Further, the correction values I′ of the (n+1)thand the (n−1)th nozzles are updated to 1+k×1x/2. Here, I indicates thecorrection value before the change, I′ indicates the correction valueafter the change, and Ix indicates the correction value of thenon-ejectable nozzle n before the change. Further, k indicates thedistribution ratio corresponding to Ix before the change. Alternatively,it may also be possible to use the distribution ratio corresponding tothe average tone value of three nozzles before the change (non-ejectablenozzle and adjacent nozzles adjacent thereto from left and right) inplace of the distribution ratio corresponding to Ix before the change ofthe non-ejectable nozzle. That is, in a case where the nth nozzle is notejectable, it may also be possible to acquire k_ave corresponding to (I(n−1)+1 (n)+I (n+1))/3 as the distribution ratio. However, it is assumedthat I (x) is the correction value of the xth nozzle before the change.At this time, the correction value I′ (n+1) of the (n+1)th nozzle isupdated to I (n+1)+k_ave×I (n)/2. For example, it is assumed that thetone value of the non-ejectable nozzle is 36 and the tone values of theadjacent nozzles to the left and right of the non-ejectable nozzle areboth 120. In this case, it may also be possible to distribute the tonevalues to the left and right nozzles by using the distribution ratiok_ave corresponding to (36+120+120)/3=92, which is the average value ofthe three nozzles, in place of acquiring the distribution ratio kcorresponding to 36, which is the tone value of the non-ejectablenozzle. By calculating the distribution ratio by using the average tonevalue as described above, as in the example described above, it ispossible to more appropriately suppress a white streak due to noejection in a case where there is a difference in the tone value beforethe change between the non-ejectable nozzle and the adjacent nozzlesadjacent thereto from left and right.

By changing the correction values of the adjacent nozzles adjacent tothe non-ejectable nozzle from left and right as described above, thedensity (number of dots or dot size) to be in the charge of thenon-ejectable nozzle is distributed to the adjacent nozzles in a ratioin accordance with the tone value of the non-ejectable nozzle. Becauseof this, even in a case where a white streak is perceived becausecorrection is insufficient or a black streak occurs because correctionis too much on a condition that the correction value of thenon-ejectable nozzle is simply distributed to the adjacent nozzles, itis possible to appropriately suppress a white spot due to thenon-ejectable nozzle.

Further, in the present embodiment, it is possible to independentlyacquire or update each of the “correction table without the influence ofa non-ejectable nozzle”, the “correction amount in a case where anon-ejectable nozzle has occurred”, and the “nozzle position at whichnon-ejection correction is performed”.

Third Embodiment

In the first and second embodiments, a white streak is suppressed bychanging the correction table so that the correction value of anon-ejectable nozzle is allocated to the adjacent nozzles located onboth sides of the non-ejectable nozzle. In the third embodiment, thewhite streak (or change in pattern due to suppression of white streak)is made unlikely to be perceived by changing the correction values ofthe peripheral nozzles in a wider range, not only the adjacent nozzleson both sides. In the following, explanation of the contents common tothose of the preceding first and second embodiments is omitted orsimplified and in the following, different points are explained mainly.

FIG. 16 is a diagram explaining a way of thinking in the presentembodiment. In FIG. 16 , the horizontal axis of the graph on the topside represents the position of each nozzle in the nozzle row and thevertical axis represents the density (measured value describedpreviously) on the paper surface in a case where the density patch of apredetermined tone is output. In FIG. 16 , a line 1601 indicates thedensity characteristic of the entire nozzle row (in the following,called “nozzle row characteristic”). Here, it is assumed that the nozzlerow characteristic 1601 is acquired in accordance with the densitycorrection information generation processing shown by the flowchart inFIG. 6 described previously. Then, here, it is assumed that an ejectionfailure is detected in the nozzle located at the position indicated by ax mark at the time of acquisition of the nozzle row characteristic 1601.At this time, in the present embodiment, a nozzle row characteristic1603 in a case where the nozzle at the position indicated by the markbecomes a non-ejectable nozzle, which is shown by a graph on the bottomside in FIG. 16 , is generated dynamically by adding a non-ejectioninfluence template indicated by a V-shaped line 1602 on the middle sidein FIG. 16 to the nozzle row characteristic 1601. Then, from the nozzlerow characteristic generated dynamically, a correction table is createdand correction processing is performed by using the correction table.Here, the non-ejection influence template is created from, for example,a non-ejection influence acquisition chart shown in FIG. 17A. Thenon-ejection influence acquisition chart shown in FIG. 17A also includesa non-ejection detection area 1701 and a density patch area 1702, likethe density characteristic acquisition chart shown in FIG. 7 describedpreviously. However, in the density patch area 1702 of the non-ejectioninfluence acquisition chart, at several nozzle positions correspondingto the above-described “intentional non-ejectable nozzles”, a portion1703 in which all the tone values are set to “0” exists. From thescanned image obtained by reading the non-ejection influence acquisitionchart after being output from the printing module 107 by the imagesensor 108, densities of the nozzles in a range that is affected by theintentional non-ejectable nozzle, for example, densities of five nozzleson the left side and five nozzles on the right side respectively withthe non-ejectable nozzle being sandwiched in between are acquired.Further, an average density in portions that are not affected by thenon-ejectable nozzle is found. Then, by subtracting the average densityfrom each of the obtained densities of the five nozzles on the left sideand the five nozzles on the right side respectively, the non-ejectioninfluence template having the characteristic as shown in FIG. 17B isobtained. In the density patch area 1702 shown in FIG. 17A, the ninekinds of density patch in which the density varies at nine levels exist,and therefore, as shown in FIG. 17B, nine non-ejection influencetemplate having the characteristics shown by nine lines are obtained. Inthis example, the letter V of the template obtained from the patch whosedensity is the lowest is the shallowest and the letter V of the templateobtained from the patch whose density is the highest is the deepest.Then, it is meant that the shallower the letter V, the smaller thedensity reduction amount is and the deeper the letter V, the larger thedensity reduction amount is. FIG. 17B is an example and for example,there is also a case where the letter V becomes shallower as the tonevalue of the density patch becomes larger with a certain density patchbeing taken as a boundary. The non-ejection influence template thusobtained is associated with the tone value of each of the originaldensity patches and stored in the external storage device 15.

By performing the correction processing by using the correction tablecreated based on the nozzle row characteristic including the influenceof a non-ejectable nozzle as described above, the number of dots (or thedot size) increases by the amount corresponding to the influence in theperipheral nozzles of the non-ejectable nozzle, and therefore, the whitestreak due to the occurrence of the non-ejectable nozzle is suppressed.In the example in FIG. 17A, the intentional non-ejectable nozzles areprovided at predetermined intervals, but it may also be possible toprovide them randomly.

<Processing Flow of Entire System>

Next, a flow of entire processing in the image forming system 10according to the present embodiment is explained with reference to aflowcharts of FIGS. 18A and 18B. As shown in FIGS. 18A and 18B, thisprocessing flow is roughly divided into three pieces of processing, thatis, nozzle row characteristic acquisition processing (S1801 to S1806),non-ejection influence template creation processing (S1807 to S1812),and printing processing (S1813 to S1821). In the following, a detailedexplanation is given for each piece of processing.

«Nozzle Row Characteristic Acquisition Processing»

The nozzle row characteristic acquisition processing is performed by thedensity correction information generation unit 305 within the imageprocessing module 106. Each piece of processing at S1801 to S1805corresponds to that at each of S601 to S605 in the flowchart in FIG. 6showing the flow of the density correction information generationprocessing of the first embodiment and there is no difference inparticular, and therefore, explanation is omitted. At S1806 in a casewhere no non-ejectable nozzle is detected, the density characteristic(see the nozzle row characteristic 1601 described previously) of eachnozzle row configuring the print head is acquired. That is, at thisstep, different from the case of S606 of the first embodiment, only theacquisition of the measured curve (see the measured curve 902 describedpreviously) of each nozzle configuring the nozzle row is performed andthe creation of the correction table is not performed.

By the processing so far, the nozzle row characteristic not includingthe influence of a non-ejectable nozzle is acquired. In a case where theacquisition of the nozzle row characteristic is completed, theprocessing moves to the non-ejection influence template creationprocessing.

«Non-Ejection Influence Template Creation Processing»

The non-ejection influence template creation processing is performed bya non-ejection influence information generation unit (not shownschematically) within the image processing module 106. Each piece ofprocessing at S1807 to S1811 is similar to that at each of S1801 toS1805 in the above-described nozzle row characteristic acquisitionprocessing. That is, the data of the non-ejection influence acquisitionchart image for which halftone processing has been performed istransmitted to the printing module 107 from the density correctioninformation generation unit 305 along with printing instructions thereofand the non-ejection influence acquisition chart is output (S1807).Next, in a case where a scanned image of the output non-ejectioninfluence acquisition chart is acquired by the image sensor 108 (S1808),non-ejectable nozzle detection processing whose target is thenon-ejection detection area of the scanned image is performed (S1809).The, in a case where a non-ejectable nozzle is detected (Yes at S1810),cleaning processing is performed (S1811) and in a case where anon-ejectable nozzle is no longer detected, the processing advances toS1812. On a condition that the environment and the conditions aredifferent between the time of non-ejection influence template creationand the time of nozzle row characteristic acquisition, there is a casewhere it becomes necessary to perform additional processing forabsorbing the difference in the application of the created non-ejectioninfluence template. Because of this, it is desirable to perform theoutput and its reading of the non-ejection influence acquisition chartin the same environment and under the same conditions as those at thetime of the output and its reading of the non-ejection influenceacquisition chart.

At S1812 in a case where no non-ejectable nozzle is detected, based onthe scanned image of the non-ejection influence acquisition chart readat S1808, a non-ejection influence template is created by the methoddescribed previously.

By the processing so far, the non-ejection influence template indicatingthe influence exerted by the non-ejectable nozzle on the densitycharacteristics of the peripheral nozzles thereof is obtained. It isnecessary to complete the processing at S1801 to S1082 before the startof the printing processing, to be described later, and for example, theprocessing at S1801 to S1812 is performed at the time of shipment of thesystem and at the time of attachment of the print head. Further, it mayalso be possible to design a configuration in which the nozzle rowcharacteristic and the non-ejection influence template are updated byperforming the processing at S1801 to 1812 again at the time of printhead exchange, or in accordance with explicit instructions from a user,or each time a predetermined number of sheets is printed or apredetermined time elapses. Furthermore, it may also be possible toevaluate the correction table that is created at S1816, to be describedlater, each time a predetermined number of sheets is printed or apredetermined time elapses and in a case where it is determined thatcorrection is insufficient, perform the processing at S1801 to S1812again and perform updating.

«Printing Processing»

Each piece of processing at S1813 and S1814 corresponds to that at S1001and S1002 respectively in the flowchart in FIG. 10 showing the flow ofthe printing processing of the first embodiment. That is, the printingpreparation processing and the non-ejectable nozzle detection processingare performed. The contents of both pieces of processing are alreadyexplained, and therefore, their explanation is omitted.

Next, at S1815, by the correction processing unit 302, processing toapply the non-ejection influence template to the nozzle rowcharacteristic acquired at S1806 is performed based on the nozzleposition number of the non-ejectable nozzle detected at S1814. Here, byusing a specific example, the template application processing isexplained in detail. FIG. 19 is a diagram showing an example of thenon-ejection influence template in the table form, which is created atS1812. Each column in the table shown in FIG. 19 indicates a relativepositional relationship from the non-ejectable nozzle and each rowindicates the tone value of the corresponding density patch. The case ofthe non-ejection influence template shown in FIG. 19 indicates that, forexample, on the density patch whose tone value is “64”, the density ofthe nozzle to the left of the non-ejectable nozzle (nozzle whoserelative nozzle position is “−1”) is a value that is obtained bysubtracting 0.148 from the density in a case where there is nonon-ejection. In the non-ejection influence template in FIG. 19 , therange that is affected by the non-ejectable nozzle is up to threenozzles on the left and three nozzles on the right of the non-ejectablenozzle respectively. The reason is that the influence that is exerted bythe non-ejectable nozzle on the nozzle positions whose relative nozzleposition is +4 or more distant or −4 or more distant is 0.001 or less,and therefore, the influence can be determined to be sufficientlyignorable. FIG. 20A is a diagram showing an example of the nozzle rowcharacteristic acquired at S1806 in the table form. In the table shownin FIG. 20A, each column corresponds to the nozzle position numberindicating the absolute position of each nozzle and each row correspondsto the tone value of the density patch. In a case of the nozzle rowcharacteristic shown in FIG. 20A, on the density patch whose tone valueis “64”, the density corresponding to the nozzle whose nozzle positionnumber is “97” is “0.472”. Here, it is assumed that the nozzle whosenozzle position number is “97” is detected as a non-ejectable nozzle bythe non-ejectable detection processing at S1814. At this time, thenozzle position number “97” is the “relative nozzle position “0”” in thenon-ejection influence template in FIG. 19 . In this case, to each valuestored in the column corresponding to the nozzle position number “97” inthe nozzle row characteristic table in FIG. 20A, each value stored inthe column corresponding to the “relative nozzle position “0”” in thenon-ejection influence template shown in FIG. 19 is added. Due to this,the density in the nozzle in a case where the nozzle whose nozzleposition number is “97” becomes a non-ejectable nozzle is found.Similarly, by adding each value stored in the column corresponding tothe “relative nozzle position n” in FIG. 19 to each value stored in thecolumn corresponding to the nozzle position number “97+n”, the densityin the nozzle in a case where the nozzle whose nozzle position number is“97+n” becomes a non-ejectable nozzle is found. In a case where theresults of the addition such as this are less than “0”, it is sufficientto set the value of the nozzle after the template is applied to “0”.FIG. 20B shows the nozzle row characteristic after the non-ejectioninfluence template in FIG. 19 is applied to the nozzle rowcharacteristic in FIG. 20A. For example, in the column whose nozzleposition number is “97”, as regards the tone value “32”, the value“0.228” before the application has changed to the value “0.116” afterthe application, which is obtained by adding “−0.112” to “0.228”.Similarly, as regards the tone value “64”, the value “0.472” before theapplication has changed to the value “0.151” after the application,which is obtained by adding “−0.321” to “0.472”. Then, similar changesoccur in the adjacent nozzles in the range of −3 to +3 with the nozzleposition number “97” being taken as a center (that is, six nozzles whosenozzle position numbers are “94 to 96” and “98 to 100”.

At S1816, by the density correction information generation unit 305, acorrection table is created based on the nozzle row characteristic afterthe non-ejection influence template is applied. The specific processingcontents are the same as those at S803 and S804 in the flow in FIG. 8 ofthe first embodiment. That is, first, the target characteristic 903 isacquired and from this, the target density value 905 corresponding tothe input tone value 904 is found, and the tone value corresponding tothe target density value 905 is determined from the measured curve 902of the nozzle of interest, and the tone value is taken as the correctionvalue (output tone value) 906 of the nozzle of interest. At this time,for the peripheral nozzle that is affected by the non-ejectable nozzle,the correction value is derived by using the nozzle row characteristicthat takes into consideration the influence of the non-ejectable nozzle.For example, in a case where the tone value whose target density valueis “0.2” is taken as the correction value, on a condition that thenozzle row characteristic not including the influence of thenon-ejectable nozzle shown in FIG. 20A is used, the correction value atthe nozzle position number “96” is “27”. On the other hand, the use ofthe nozzle row characteristic after the template in a case where thenozzle whose nozzle position number is “97” is the non-ejectable nozzleis applied will result in that the correction value at the nozzleposition number “96” is “35”. As described above, for the nozzle that isaffected by non-ejection, a large correction value is obtained comparedto a case where there is no non-ejection. Then, by the correction valueof the nozzle that is affected by non-ejection becoming large, thenumber of dots ejected in the nozzle increases (or the dot sizeincreases) and as a result of that, the influence of the w % bite streakdue to the non-ejectable nozzle is suppressed.

S1817 to S1821 correspond to S1004 to S1008 respectively in theflowchart in FIG. 10 showing the flow of the printing processing of thefirst embodiment and there is no difference in particular, andtherefore, their explanation is omitted.

The above is the flow of the entire processing in the image formingsystem 10 according to the present embodiment. By the series of printingprocessing including S1814 to S1816 described above, the white spot thatoccurs due to a non-ejectable nozzle that has occurred before printingand a non-ejectable nozzle that occurs during printing is suppresseddynamically. Then, even in a case where a non-ejectable nozzle occursnewly or a non-ejectable nozzle recovers during printing, it is notnecessary to repeat again the nozzle row characteristic acquisitionprocessing and the non-ejection influence template creation processing,and therefore, this also leads to suppression of the system downtime.

Modification Example

In the above-described example, it is supposed that the detection of anon-ejectable nozzle is performed online in accordance with the numberof output sheets also during printing and the non-ejectable nozzledetection processing (S1814) is incorporated in the series of processingconfiguring the printing processing. However, the non-ejectable nozzledetection timing is not limited to this and for example, theconfiguration may be one in which the non-ejectable nozzle detectionprocessing is performed between the non-ejection influence templatecreation processing (S1812) and the printing preparation processing(S1813).

Further, it may also be possible not to perform ink ejection for anon-ejectable nozzle by setting all the correction values to “0” afterthe correction table is created. Alternatively, it may also be possibleto prohibit ink ejection by setting all the threshold valuescorresponding to the non-ejectable nozzle in the threshold value matrixused in the halftone processing to “255” or more, setting the input tonevalue thereof to “0”, masking the pixel value of the HT image to “0”,and so on. By doing so, it is possible to suppress a black streak or thelike from occurring in a case where a non-ejectable nozzle recoversunexpectedly during printing.

Further, the non-ejection influence template may be one that isindicated by a W-shaped line like a line 1602′ in FIG. 16 . In thiscase, it is possible to obtain the non-ejection influence template bysetting the density reduction amount at the non-ejectable nozzleposition (relative nozzle position=0) to “0” and changing the densityreduction amounts of the peripheral nozzles thereof so that the areaindicated by the line 1602 and the area (that is, integral value)indicated by the line 1602′ are equal. For example, it may also bepossible to distribute the density reduction amount at the non-ejectablenozzle position in accordance with the ratio of the density reductionamount at each of other nozzle positions. Alternatively, it may also bepossible to create the non-ejection influence template after applying apublicly known spatial filter to the nozzle row characteristic. Further,in the above-described example, the non-ejection influence template isstored in the table form, but it may also be possible to approximate theinfluence by the non-ejectable nozzle by a function (for example,Gaussian function) and store the functional formula thereof.

In the above-described example, the nozzle row characteristic and thenon-ejection influence template are prepared and stored before theprinting processing and the correction table is created based on thenozzle row characteristic after the non-ejection influence template isapplied, but this is not limited. For example, the configuration can beone in which a correction table and a non-ejection influence templatefor the correction table are stored in advance and the non-ejectioninfluence template is added with the detected non-ejectable nozzle beingtaken as a center. In place of the configuration in which thenon-ejection influence template is added, the configuration can also beone in which a nozzle row characteristic that suppresses the influenceof non-ejection is stored in advance as a template and in a case where anon-ejectable nozzle occurs, the non-ejection influence template isreplaced with the template. However, in the configuration in which thetemplate is replaced with another, the density correction amounts forsuppressing the density unevenness, which should be different for eachnozzle, are overwritten by the template. In particular, for the nozzledistant from the non-ejectable nozzle, the influence of non-ejection issmall and priority should be given to the density unevenness correction,but in a case where overwrite by the template is performed, the effectis no longer obtained. Further, there is a case where anothernon-ejectable nozzle occurs within the range that is affected bynon-ejection. In order to deal with this case by the configuration inwhich the template is replaced with another, it is necessary to storethe template for each positional relationship of a plurality ofnon-ejectable nozzles. In this regard, with the configuration in whichthe template is added, it is only required to apply the template to eachposition at which a non-ejectable nozzle has occurred, and therefore, itcan be said that the configuration in which the template is added issuperior to the configuration in which the template is replaced withanother.

Further, there is a case where a non-ejectable nozzle does not recovereven by performing cleaning processing repeatedly. In this case, it mayalso be possible to create a correction table not including theinfluence of non-ejection by reversing the signs (plus and minus) of thenon-ejection influence template and adding the non-ejection influencetemplate to the nozzle row characteristic.

Fourth Embodiment

In the third embodiment, the density characteristic acquisition chartand the non-ejection influence acquisition chart are separate charts.Further, both charts include the non-ejection detection area and thenon-ejection detection processing is performed based on the results ofreading of the area (S1803, S1809). Next, an aspect is explained as thefourth embodiment in which the processing up to the acquisition of anozzle row characteristic and the creation of a non-ejection influencetemplate is performed by using a common chart not including anon-ejection area without performing non-ejection detection processing.In the following, explanation of the contents common to those of thepreceding first to third embodiments is omitted or simplified and in thefollowing, different points are explained mainly.

Three charts 2101 to 2103 shown in FIG. 21 are examples of the chartsthat are used in the present embodiment. Each chart image is created sothat the non-ejection detection area 1701 shown in FIG. 17A describedpreviously is not included and the position of the intentionalnon-ejectable nozzle is different between each chart image. Further,each of three lines 2111 to 2113 in FIG. 21 shows part of a nozzle rowcharacteristic obtained from each scanned image of the three printed andoutput charts 2101 to 2103 described above. A x mark in FIG. 21indicates the position corresponding to the intentional non-ejectablenozzle. Here, it is obvious that in a case where a non-ejectable nozzleoccurs, the densities of the nozzle and the peripheral nozzles becomelow (become bright). Consequently, by selecting the highest densitycharacteristic for each nozzle from the plurality of nozzle rowcharacteristics obtained from the output results of the plurality ofcharts, it is possible to obtain the nozzle row characteristic notincluding the influence of a non-ejectable nozzle. Alternatively, it isalso possible to obtain the nozzle row characteristic not including theinfluence of a non-ejectable nozzle by averaging a number of densitiesless than the number of charts in order from the highest density foreach nozzle. It may also be possible to determine the number ofdensities to be averaged based on the probability of non-ejectablenozzle occurrence and for example, it may be possible to take themaximum number of output sheets with which a non-ejectable nozzle is nolonger included statistically with a probability of 99% or higher as thenumber of densities to be averaged. By selecting and calculating thedensity characteristic for each nozzle from the output results ofdifferent charts as described above, it is also possible to exclude theinfluence of the unintentional non-ejectable nozzle. For example, it isassumed that a non-ejectable nozzle that is not generated intentionallyhas occurred at the position indicated by a “!” mark of a line 2113′ inFIG. 21 . In this case also, a nozzle row characteristic not includingthe influence of a non-ejectable nozzle is obtained by finding thenozzle density characteristic by using one of the line 2111 and the line2112, or the average value of the curves 1901 and 1902.

Further, it is possible to create the non-ejection influence template byperforming the processing described at S1812 described previously to theplurality of the charts 2101 to 2113 as shown in FIG. 21 . However, in acase where an unintentional non-ejectable nozzle has occurred within therange that is affected by an intentional non-ejectable nozzle, thenon-ejection influence template is created by excluding the intentionalnon-ejectable nozzle. For example, it is sufficient to exclude theintentional non-ejectable nozzle in a case where another vertexindicated by the x mark exists within the range that is affected by theintentional non-ejectable nozzle as on a line 2111′ in FIG. 21 .

By the method as described above, it is also possible to obtain the sameeffect as that of the third embodiment.

Fifth Embodiment

In the first to fourth embodiments, an attempt is made to suppress awhite streak that occurs in a case where a non-ejectable nozzle occursby distributing the density in the charge of the non-ejectable nozzle tothe peripheral nozzles. In the fifth embodiment, processing is performedso that the shape of an object within a printing-target image is notimpaired, in addition to suppression of a white streak. In thefollowing, explanation of the contents common to those of the precedingfirst to fourth embodiments is omitted or simplified and in thefollowing, different points are explained mainly.

FIG. 22 is a flowchart showing a flow of printing processing accordingto the present embodiment. This flowchart differs from the flowchart inFIG. 10 according to the first embodiment lies in that a step ofexchanging pixel values (S2203) is inserted between the step ofdetecting a non-ejectable nozzle (S2202) and a step of changing thecorrection table (S2204). That is, S2201 and S2202 in the flowchart inFIG. 22 correspond to S1001 and S1002 described previously and S2204 toS2209 correspond to S1003 to S1008, respectively, and there is nodifference in particular. However, in the correction processing at 2205,the printing-target image after the above-described pixel value exchangeis performed is the target thereof. In the following, the pixel valueexchange processing, which is the feature of the present embodiment, isexplained in detail.

<Pixel Value Exchange Processing>

The pixel value exchange processing is performed by a pixel valueexchanging unit (not shown schematically) within the image processingmodule 106. FIG. 23 is a flowchart showing details of the pixel valueexchange processing at S2203. In the following, explanation is givenalong FIG. 23 .

At S2301, based on the non-ejectable nozzle information generated atS2202, the pixel line corresponding to the non-ejectable nozzle ofinterest among the detected non-ejectable nozzles is specified from eachpixel line of the printing-target image.

Next, at S2302, the tone value (pixel value) of the pixel of the pixelline specified at S2301 and the pixel value of the pixel linecorresponding to the nozzle adjacent to the non-ejectable nozzle ofinterest from left or right (in the following, called “alternativenozzle”) are compared. At this time, it is preferable to fix thealternative nozzle to the right or left of the non-ejectable nozzle ofinterest irrespective of the nozzle position number and exchange pixelvalues preferentially in one direction. In a case where the results ofthe comparison indicate that the pixel value of the pixel linecorresponding to the non-ejectable nozzle of interest is larger, theprocessing advances to S2303 and the compared pixel values adjacent toeach other are exchanged. On the other hand, in a case where the pixelvalue of the pixel line corresponding to the non-ejectable nozzle ofinterest is less than or equal to the pixel value of the pixel line ofthe alternative nozzle, the processing advances to S2304.

At S2304, whether or not the comparison of pixel values and the exchangeof pixel values are completed by taking all the alternative nozzles (n)as a target is determined. In a case where there is an unprocessedalternative nozzle, the processing returns to S2302 and the comparisonof pixel values and the exchange of pixel values are repeated byupdating n and taking the next alternative nozzle as a target. In therepetition processing, the pixel value of the pixel line correspondingto the non-ejectable nozzle of interest, which is compared at S2302, isthe pixel value after being exchanged at immediately subsequent S2303.On the other hand, in a case where the comparison of pixel values andthe exchange of pixel values are completed by taking all the alternativenozzles (n) as a target, the processing advances to S2305.

At S2305, whether or not the processing at S2302 to S2304 describedabove are completed by taking all the non-ejectable nozzles included inthe non-ejectable nozzle information generated at S2202 as a target isdetermined. In a case where there is an unprocessed non-ejectablenozzle, the processing returns to S2301, and the next non-ejectablenozzle of interest is determined and the processing at S2302 to S2304described above is repeated. On the other hand, in a case where theprocessing is completed for all the non-ejectable nozzles, this flow isterminated.

The above is the contents of the pixel value exchange processing. Bythis processing, it is possible to prevent an object, such as a thinline, which is to be formed by the non-ejectable nozzle, from beingimpaired. In a case where the print head is a so-called multi-columnhead consisting of nozzle rows comprising nozzles performing drawing atthe same position in an overlapping manner, it is also possible to applythe present embodiment. In this case, it is sufficient to perform theabove-described processing by taking the nozzle that performs drawing atthe same position in the nozzle row different from the nozzle row towhich the non-ejectable nozzle belongs as the alternative nozzle of thenon-ejectable nozzle.

OTHER EMBODIMENTS

In each embodiment, as the nozzle characteristic, the densitycharacteristic is used and the correction table is also created based ondensity, but for example, it may also be possible to use thecharacteristic of Y of the CIEXYZ color space or L* of CIELab* for eachnozzle and create the correction table also based on the characteristic.

In a case where the scan resolution of the image sensor 108 is differentfrom the resolution of nozzle arrangement of the print head, it ispreferable to perform conversion so that the resolution of the scannedimage coincides with the resolution of nozzle arrangement. Forconversion, it is possible to use a publicly known interpolation methodand it is possible to perform enlargement or reduction to the resolutionof nozzle arrangement by using, for example, the nearest neighbormethod, the bilinear method, the bicubic method and the like.

Further, in each embodiment, the example is explained in which thedensity correction processing is performed by taking the tone value ofthe printing-target image as a target, but a configuration may beaccepted in which the density correction processing is performed bytaking the threshold value of the threshold matrix as a target based onthe same way of thinking. With this configuration, it is also possibleto obtain the same effect of each embodiment.

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

According to the technique of the present disclosure, it is possible toimplement highly accurate density unevenness correction whilesuppressing a reduction in productivity of printing accompanyingcorrection value calculation for density unevenness correction.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2020-063628 filed Mar. 31, 2020, which is hereby incorporated byreference wherein in its entirety.

What is claimed is:
 1. An image processing apparatus for an imageforming apparatus that performs printing on a printing medium by using aprint head comprising a nozzle row in which a plurality of nozzlesejecting ink is arrayed in one direction, the image processing apparatuscomprising: one or more processors; and one or more memories that storecomputer-readable instructions for causing, when executed by the one ormore processors, the image processing apparatus to function as: adetection unit configured to detect a non-ejectable nozzle that cannoteject ink normally among nozzles configuring the nozzle row; anacquisition unit configured to acquire a nozzle row characteristicindicating a characteristic of each nozzle based on a scanned image of afirst chart including a plurality of patches, which is output from theimage forming apparatus; a creation unit configured to create a templateindicating influence exerted by the non-ejectable nozzle based on ascanned image of a second chart including a plurality of patches, whichis output from the image forming apparatus; and a generation unitconfigured to generate density correction information for reducingdensity unevenness in the printing, wherein the first chart and thesecond chart include a plurality of patches having different tones andextending in a direction of the nozzle row, each patch of the secondchart includes at least a portion in which a dot is not formed at aposition corresponding to a specific nozzle in the nozzle row, thegeneration unit: acquires the nozzle row characteristic including theinfluence of the non-ejectable nozzle by applying the template inaccordance with a position of the non-ejectable nozzle detected by thedetection unit to the nozzle row characteristic acquired by theacquisition unit; and generates density correction information thatspecifies an output tone value for implementing a target density for aninput tone value for each nozzle based on the acquired nozzle rowcharacteristic including the influence of the non-ejectable nozzle thesecond chart further includes a pattern for detecting the non-ejectablenozzle, the detection unit detects the non-ejectable nozzle by analyzingthe pattern in the scanned image of the second chart that is output fromthe image forming apparatus, and the creation unit creates the templatebased on the non-ejectable nozzle detected by the detection unit.
 2. Theimage processing apparatus according to claim 1, wherein the generationunit: acquires the nozzle row characteristic including the influence ofthe non-ejectable nozzle detected by the detection unit by adding thetemplate to the nozzle row characteristic acquired by the acquisitionunit; and generates the density correction information from the acquirednozzle row characteristic.
 3. The image processing apparatus accordingto claim 2, wherein the characteristic of each nozzle specified by thenozzle row characteristic is a density characteristic, in the template,a relative nozzle position for the non-ejectable nozzle and a densityreduction amount are associated with each other, and the generationunit: finds a nozzle row characteristic whose density is reduced by theassociated density reduction amount based on a relative positionalrelationship with the nozzle detected as a non-ejectable nozzle by thedetection unit for each nozzle whose density characteristic is specifiedby the nozzle row characteristic acquired by the acquisition unit; andgenerates the density correction information from the found nozzle rowcharacteristic.
 4. An image processing apparatus for an image formingapparatus that performs printing on a printing medium by using a printhead comprising a nozzle row in which a plurality of nozzles ejectingink is arrayed in one direction, the image processing apparatuscomprising: one or more processors; and one or more memories that storecomputer-readable instructions for causing, when executed by the one ormore processors, the image processing apparatus to function as: adetection unit configured to detect a non-ejectable nozzle that cannoteject ink normally among nozzles configuring the nozzle row; a creationunit configured to create a nozzle row characteristic indicating acharacteristic of each nozzle and a template indicating influenceexerted by the non-ejectable nozzle based on scanned images of aplurality of charts output from the image forming apparatus; and ageneration unit configured to generate density correction informationfor reducing density unevenness in the printing, wherein each of theplurality of charts includes a plurality of patches having differenttones and extending in a direction of the nozzle row, each patchincludes at least a portion in which a dot is not formed at a positioncorresponding to a specific nozzle in the nozzle row, the portion inwhich a dot is not formed is different for each of the plurality ofcharts, the generation unit: acquires a nozzle row characteristicindicating a characteristic of each nozzle from each scanned image ofthe plurality of charts; generates a nozzle row characteristic notincluding influence of a non-ejectable nozzle by selecting a highestdensity characteristic for each nozzle from a plurality of acquirednozzle row characteristics, or averaging a predetermined number ofdensities in order from the highest density for each nozzle; acquiresthe nozzle row characteristic including the influence of thenon-ejectable nozzle by applying the template in accordance with aposition of the non-ejectable nozzle detected by the detection unit tothe generated nozzle row characteristic not including the influence ofthe non-ejectable nozzle; and generates density correction informationthat specifies an output tone value for implementing a target densityfor an input tone value for each nozzle based on the acquired nozzle rowcharacteristic including the influence of the non-ejectable nozzle.